Plasmatrons use an electric arc to generate a stream of high temperature gas and are currently used in many applications, including the attachment to various plasma torch and plasma nozzle configurations used for plasma spraying. In a plasmatron, a plasma forming gas flows through an arc chamber and a plasma stream is generated by an electric arc formed between a cathode and an anode generally located at the opposite sides of the arc chamber. In prior art designs, the arc root attachment and the plasmatron operating stability are dependent upon the plasma gas flow rate, gas pressure or electrical power operation. In practical use of plasmatrons, the gas flow rate, gas pressure or electrical power conditions do vary accidentally. Such accidental variations induce uncontrolled changes in the properties of the plasma stream.
Generally, a constant gas flow rate through the plasmatron has to be controlled as means to maintain a stable arc length and avoid excessive arc root fluctuations. However, in practice, even when the plasma gas flow is strictly controlled, the arc root displays occasional and unpredictable longitudinal excursions with deleterious effects for the application of the plasma torch.
It is known that the use of high voltage-low amperage arcs to operate a plasma torch has certain advantages in terms of reduced electrode wear and improved thermal efficiency of the torch. Since Ohm's law apply to a plasma stream, in order to increase the voltage of the arc the experimenter can use various means such as: (a) increase the electrical resistivity of the plasma gas by increasing the pressure, by increasing the gas flow, by using plasma gasses with higher electrical resistivity or by constricting the plasma gas flow, (b) extend the length of the arc.
The influence of arc channel diameter on the electric field is described by the K.sub.E criterion as derived from Ohm's law: EQU K.sub.E =(.sigma..times.E.times.d.sup.2).times.I.sup.-1
where:
"E" is the electric field strength, "d" is the diameter of the arc channel, "I" is the arc current and ".sigma." is the specific electrical conductance of the plasma gas.
The electric field dependents exponentially with the diameter "d" as it varies with its square power. Theoretically, a long and narrow arc channel should lead to higher arc voltages. The practical impediment is to prevent the arc from attaching randomly to the internal wall of the channel or from random axial excursions of the arc root. The axial excursions can be partially reduced by increasing the gas flow or gas pressure and shortening the arc length. This would be uneconomical and to the detriment of plasma torch efficiency. Another practical impediment relates to maintaining a stable arc root attachment and therefore a stable arc length when water-cooling, plasma gas flow, plasma gas pressure or the power application vary either accidentally or on purpose by the torch operator.
An approach to control the arc root attachment within a straight plasma duct is found in U.S. Pat. No. 4,841,114 and U.S. Pat. No. 4,916,273 of Browning. Browning discloses a singular surface discontinuity formed at a downstream position along a constant cross-section anode nozzle bore, the discontinuity being in the form of a groove, an annular shoulder, a counterbore or an output shoulder. The discontinuity is meant to prevent the migration of the arc root towards the end of the anode nozzle exit and to induce wear of the anode nozzle exit. It is apparent that this design operates at significantly higher gas flow rates. The high gas flow pushes the arc forward therefore extending the arc linearly while the discontinuity is claimed to prevent the arc root attachment from migrating further downstream of the discontinuity. The attachment of the arc root in a surface groove as shown in Browning, if that is even possible, may in itself lead to unpredictable instabilities associated with the gas turbulence developed within the groove channel. Variations in the cooling rate of the anode nozzle bore or variations in the gas flow rate may easily determine the arc root to escape the effect of the discontinuity and therefore migrate substantially along the axis, even if only for a short time. Such instabilities affect the parameters of the plasma stream, which then will affect negatively the quality and repeatability of the plasma sprayed coatings. The high gas flow rates required to operate the Browning designs will induce high operating costs of the plasma spray torch.
A different approach to control arc root attachment to a remote annular and smooth anode is found in U.S. Pat. No. 5,332,885 of Landes which discloses a plurality of cathodes generating a plurality of arcs within a common arc chamber, the arcs attaching to a common anode bore. An intermediate section comprises a plurality of electrically neutral annular rings, which Landes refers to as "neutrodes". The apparatus disclosed by Landes is very complicated and the plurality of arcs will interfere with each other resulting in an unstable torch operation. Even if Landes used only one cathode, when an ionized plasma is generated, the neutrode rings act as electric capacitors therefore attaining an electric charge on their inner surface. This results in arching to the rings due to secondary electrode effect, therefore deleteriously affecting the functioning of the plasmatron. A similar approach towards the use of electrically floating segmented anodes is described in U.S. Pat. No. 5,900,272 of Goodman.
U.S. Pat. No. 5,296,668 of Foreman et al. teaches a gas cooled cathode, electrically insulated by means of an insulating collar and operating in conjunction with an elongated and smooth anode tube having a small conical entrance portion. This design also relies on the gas flow rate and sufficient cooling of the anode nozzle bore to push the arc and force a downstream migration and a random attachment of the arc root. There are no provisions to stabilize the arc root location and the arc root will migrate longitudinally without any means to control it effectively.
Other prior art discloses the use of gas flow constrictors to increase gas resistivity and raise the arc voltage as well as the use of electrically insulating sleeves within the arc chamber to extend the arc and avoid arching to the chamber wall. Such prior art is found in U.S. Pat. No. 4,882,465 of Smith et al., U.S. Pat. No. 5,008,511 of Ross, U.S. Pat. No. 5,420,391 of Delcea, and U.S. Pat. No. 5,514,848 of Ross et al. Identical arc constrictor like that disclosed by Ross et al. is also disclosed in Soviet Union Patent SU No. 1623846 of Granovski wherein the arc is pushed by the gas through the constrictor and is transferred to the workpiece which is positively biased. U.S. Pat. No. 4,317,984 of Fridlyand discloses a plasma torch apparatus comprising a plasmatron method whereby an arc generated at the cathode tip is pushed through a first constrictor located close to the cathode tip and is transferred further to an anode counterbore positioned downstream of a second constrictor. This arrangement functions only with additional plasma trimmer or support gasses which are introduced in the annular space between the first and the second constrictors, therefore it is too complicated and without any apparent benefit to stabilizing the arc root attachment. Both constrictors disclosed by Fridlyand have relatively large cross-section and function as means to transfer the arc into the counterbore by acting mainly as arc column guides.
Smith, Ross et al., Delcea and Granovski disclose the general use of constrictors in the gas flow passage with the gas flow acting to effectively push the electric arc through the throat of the constrictor. This leads to a reduction in the amperage to voltage ratio (A/V) of less than it would be desirable and further, the designs are sensitive to variations in the gas flow. In Ross et al. and in Delcea the arc is pushed through the throat of the constrictor by the velocity of the gas sufficiently to pass through the throat and to attach to a smooth cylindrical surface of an anode electrode, positioned relatively shortly downstream of the exit of the constrictor. In order to achieve this effect, a high ratio of gas flow to power application would be necessary to prevent arc attachment to the constrictor and fluctuations in the torch electrical operation. The arc root is left to fluctuate axially in an uncontrolled and unpredictable manner. In Ross et al. the stable functioning is disclosed as being dependent on the given power application to the electrodes and the work parameters of the electrode structure will therefore vary with any variations in the power application while theoretically, the arc voltage attainable by such a design is expected to be significantly below 200V.
The inventor found that the gas flow to power ratio is an important parameter of a plasmatron, particularly when used for plasma spraying. This parameter is indicative of enthalpy or in other words the heat content per unit of plasma gas, measured for example in kJ/mole of gas. The higher the enthalpy, the more heat is available in the plasma gas to melt the powder. When low plasma gas flows are used in conjunction with a high voltage-high power arc, higher enthalpy plasma streams are generated, and superior coatings can be plasma sprayed. The difficulty in generating a stable high arc is not with respect to stretching and constricting the arc which are readily achievable by the appropriate shape and length of the arc chamber wall, instead, the difficulty is in maintaining a stable arc length and controlling the axial movement of the arc root attachment.
The prior art offers only a limited degree of control over the arc length stability and are therefore subject to unpredictable arc root longitudinal excursions. In the prior art designs, the gas flow rate and power application to the plasmatron play a significant part in controlling both the arc length and the amperage to voltage ratio as well as to prevent the excessive axial movement of the arc root on the anode surface. In addition, the high gas flow to power ratios required to operate prior art plasmatrons lead to lower enthalpy and lower plasma spray efficiency.
There are plasma spray torches claimed to apply a plasma spray coating inside of small diameter pipes wherein a very short plasma arc is generated between a cathode tip and the nozzle bore. Such prior art plasma torches generate a low voltage, low power and weakly ionized plasma stream into which the powder is injected and therefore are known to be very inefficient. Examples are found in U.S. Pat. No. 4,970,364 of Muller, U.S. Pat. No. 4,661,682 of Gruner et al. and U.S. Pat. No. 5,837,959 of Muelberger et al. It would be desirable to employ the use of a higher ionized plasma stream to improve coating quality. Whenever such plasma spray torches require a plasma of larger magnitude than the plasma generated by one plasmatron, a desired plurality of plasmatrons can be arranged within a single plasma torch apparatus which combines the pluralities of plasmas into a single applicable plasma stream. Examples are found in U.S. Pat. No. 5,008,511 of Ross, U.S. Pat. No. 3,140,380 of Jensen, U.S. Pat. No. 3,312,566 of Winzeler et al. and U.S. Pat. No. 5,556,558 of Ross et al. A schematic example of such multiple use of plasmatrons in converging relationship is also found at page 31 of a Russian Book by Donskoi et al., Leningrad, 1979. Patent '511 teaches the use of "C" shaped and "D" shaped cross-sections applicable to a plurality of plasma channels converging into a common plasma spray output nozzle.
The majority of plasma spray apparatuses inject plasma spray material in a plasma stream exhibiting little or no ionization. The only apparatus which apparently could generate a somehow higher ionized plasma stream is disclosed in the cited prior art by Browning. However, Browning claims that the method and apparatus thereof is meant to inject powder into the hot gas exhibiting no ionization. U.S. Pat. No. 4,788,402 of Browning, teaches the benefits of injecting spray material into an expanded ionized flame but the apparatus described therein uses tremendously high quantities of expensive plasma gas at a very high pressure of about 170 lb/in.sup.2 (.about.1,200 kPa), while attaining an optimum working arc voltage of only 180-190V. These working conditions are not adequate to induce sufficient gas ionization of the second degree and an enhanced plasma enthalpy. The arc root attachment in patent '402 is pushed downstream by the very high gas flow and locates on the output lip of the plasma nozzle. It is well known that this arc attachment leads to a rapid deterioration of the nozzle output and practical experience has proven that in this situation, the arc is very unstable, often exiting the nozzle bore to attach on the front face of the plasma torch. Another disadvantage of the method in patent '402 is the very narrow margin of error with respect to optimum operating gas flow as disclosed therein, therefore indicating that this design operates only with a very high gas flow, which must also be strictly controlled within restrictive limits. An example of how the use of high gas flows and gas pressures can lead to a low ionized, low temperature plasma despite higher arc voltages is found in U.S. Pat. No. 5,637,242 of Muehlberger where a plasma stream temperature reported to be in the 3000.degree. K. range is practically insufficient to ionize sufficiently the plasma gas and to transfer adequate heat to the powder particles. This is a serious disadvantage for spraying high melting point materials such as ceramics. For example, the thermal conductivity of a nitrogen plasma, in other words plasma capacity to transfer heat and melt the powder particles is about 0.45 W/m.degree. K. at 3000.degree. K., about 2.8 W/m.degree. K. at 6000.degree. K. and about 5.3 W/m.degree. K. at 7000.degree. K.
It has been found by the applicant without having a complete explanation, that superior plasma spray coatings can be produced when the feedstock material is injected into a sufficiently ionized region of a plasma stream and is then confined to travel sufficiently through such an ionized region. The enhanced ionization is visible as a flame of higher intensity and a stream of powder spray material brighter than normal is projected through the plasma stream, this being indicative of superior heating and melting of the powder. It is believed that the higher arc voltage (higher than 120V and typically in the range of 200-500V) applied to lower gas flows crosses the threshold necessary to induce an enhanced plasma gas ionization of the second degree, sufficient to expand considerably the second degree ionized region of the plasma stream. Thus, a hotter plasma stream is generated with an estimated average temperature significantly in excess of 3000.degree. K. and typically higher than 5000.degree. K. Consequently, when such a plasma stream is used with a plasma spray torch, the melting of the powder material injected into a sufficiently ionized plasma stream having an enhanced enthalpy is superior to prior art plasma spray torch methods and apparatuses. mainly due to the increased heat transfer to the powder, particularly resulted from enhanced exhotermic ionic recombinations of the second degree.
It would be therefore desirable to provide a plasmatron capable of operating with a stable arc at higher voltages while using lower gas flows or gas pressures and therefore inducing higher plasma enthalpy and plasma stream temperature. It would also be desirable to provide a plasmatron generating a stable arc by controlling the arc root location on the anode electrode, with reduced influence by gas flow, gas pressure, or electrical fluctuations. It would be further desirable to provide a plasmatron operating optimally with a stable electric arc within a wider range of gas flows, gas pressures and power applications. All the above requirements for a superior plasmatron would be fulfilled if the anode arc root attachment is stabilized to the anode and the voltage fluctuations occur within controlled limits